The present invention relates to a method of and a system for operating a combined plant which is composed of a gas turbine, a waste-heat recovery boiler and a steam turbine and more particularly, to a method of and a system for operating a combined plant by making use of inlet guide vanes of the gas turbine.
Japanese Patent Examined Publication No. 60-17967 proposes a combined plant composed of a gas turbine, a waste-heat recovery boiler for collecting heat possessed by the exhaust gas from the gas turbine, and a steam turbine driven by the steam generated by the waste-heat recovery boiler. It is known that the temperature of the exhaust gas from the gas turbine is highest when the gas turbine operates with about 80% load and that the temperature of the steam generated by the waste-heat recovery boiler exhibits almost the same tendency.
The highest exhaust gas temperature from the gas turbine at about 80% load is attributable to the fact that, in general, the rate of introduction of air into the gas turbine is controlled when the load level has reached about 80% so as to prevent the combustion temperature in the gas turbine from increasing beyond a predetermined limit which is determined for the purpose of protecting the gas turbine blades and other parts from heat.
In operation of a gas turbine, air is introduced into a compressor through inlet guide vanes and the air compressed by the compressor is introduced into a combustion chamber together with a fuel so as to burn the fuel. The combustion gas generated as a result of the combustion drives the impeller of the gas turbine which produces driving power to drive the compressor and a load such as an alternator. During the operation of the gas turbine, the opening degree of the inlet guide vanes is controlled in the following manner.
FIGS. 2A-2D are graphical illustrations of operation factors of a combined plant such as temperatures of various portions of the combined plant, air flow rate, opening degree of the inlet guide vanes, and so forth, in relation to the load applied to the gas turbine. As shown in FIG. 2A, the inlet guide vane opening is maintained constant when the load on the gas turbine is 80% or less, so that the flow rate of the air at the compressor inlet is constant. It will be understood that the gas turbine which drives an alternator operates at a constant rotation regardless of a change in the load so that the flow rate of air taken into the compressor is also maintained constant insofar as the inlet guide vane opening is kept constant. On the other hand, the supply of the fuel into a gas turbine is determined in proportion to the level of the load so that the air excess ratio increases as the load level becomes lower. Consequently, the combustion temperature becomes lower because the turbine is cooled by excess air, as shown in FIG. 2C, with the result that the exhaust gas temperature from the gas turbine, as well as the steam temperature, becomes lower as shown in FIG. 2D. In general, the combustion temperature in the gas turbine reaches the maximum allowable temperature Tmax when the load on the turbine is increased to about 80%. In order to protect the parts of the combustion chamber and the turbine, it is not permissable to raise the temperature beyond the level of the maximum allowable combustion temperature. Therefore, when the gas turbine operates with a load exceeding 80%, the opening of the guide vanes is increased to allow a greater amount of air to be introduced so as to cool the gas turbine thereby maintaining the temperature below the maximum allowable temperature. Consequently, the exhaust gas temperature and, hence, the steam temperature exhibits peaks when the load is around 80% load.
This fact will be more easily understood from the following description taken in conjunction with the entropy graph of FIG. 3. Wherein A represents the state of air at the compressor inlet, B.sub.1 and B.sub.2 represent the state of air at the compressor outlet, C.sub.1 and C.sub.2 represent the state of the combustion gas at the combustion chamber outlet and D.sub.1 and D.sub.2 represent the state of the gas at the turbine outlet, respectively, in terms of entropy and temperature. Lines B.sub.1 -C.sub.1, B.sub.2 -C.sub.2, and A-D.sub.1 -D.sub.2 are constant pressure lines. The solid line shows the cycle (A-B.sub.1 -C.sub.1 -D.sub.1 -A) performed when the gas turbine operates at 100% load with the inlet guide vane opening increased to allow a large quantity of air to be introduced to the compressor, while the broken line shows the cycle (A-B.sub.2 -C.sub.2 -D.sub.2 -A) performed when the gas turbine operates at 80% load with the guide vane opening restricted to allow a smaller quantity of air to be introduced into the gas turbine. As shown in FIG. 3, a high temperature of the exhaust gas from the turbine is obtained by controlling the inlet guide opening such that the combustion temperature in the gas turbine does not exceed the allowable maximum temperature Tmax at each load level.
Thus, the temperature of the exhaust gas varies along a curve which is upwardly convexed as a result of the control of the airflow rate for the purpose of protecting the gas turbine and the steam temperature characteristic of the waste heat recovery boiler which is designed to maximize the heat recovery efficiency exhibits the same tendency, as shown in FIG. 2B.
In general, a combined plant is required to start and stop very frequently because it can be started rapidly and operate with a varying load at high efficiency. The upwardly convexed curve of the steam temperature characteristic, i.e., the presence of a peak, means that the generation of thermal stress in the steam turbine is very severe. For instance, the difference between the steam temperature during operation of the gas turbine at 80% load and that during operation at 100% load is 25.degree. to 30.degree. C. Such a change in the load level on the gas turbine is finished in quite a short period of time, e.g., several minutes.
Thus, the operation of the gas turbine in a combined plant has to be controlled taking into account also the conditions of the steam/water side of the plant.